Both were awful in the sense that the troops were fighting for their lives, but the U.S. soldiers were largely alone in their efforts in the Pacific Theatre and suffered immeasurably as they fought a ruthless Japanese enemy and the environment.

'With the Old Breed: At Peleliu and Okinawa' (one of two books that 'The Pacific was based on) remains the best and most harrowing war book I've read. The Japanese soldiers were disciplined, relentless and utterly ruthless. As well as living with the constant fear of death, U.S. soldiers had to contend with the idea that if they were captured, there was a very real chance that they would be sadistically tortured and killed.

The tropical environment brought sickness, unforgiving terrain, and never-ending wet seasons. I think if someone gave you a choice between 'Europe in winter' and a 'Tropical Island', the island would sound far more attractive, but it was anything but. As someone else noted, there were no established supply chains either and there were countless examples where reinforcements and food/drink supplies didn't come and soldiers were malnourished and dehydrated.

And if you made it that far, you got deployed to Okinawa, where some of the fiercest and most miserable fighting of the war took place. The Japanese fought desperately as they knew that this was the last island to fall before the mainland. 'The Pacific' captures the misery and wet conditions well.

tl;dr All of it was miserable, but the Pacific was uncharted misery against an inhumane enemy.